When looking through spectacles, the pair of eyes continuously executes viewing movements, whereby the visual points within the spectacle lenses are displaced. If the pair of eyes looks through two points in both spectacle lenses, which generate different prismatic secondary powers, an artificial heterophoria is generated and the fusion is strained.
If anisometropia exists (i.e., unequal far point refraction of both eyes), the spectacle lenses differ in their power. If the pair of eyes executes viewing movements in the same direction behind the spectacles, different prismatic secondary powers result—in contrast to spectacles having equal power on the left and right—in the left and the right visual points, which strain the fusion and/or the binocular vision.
The prismatic imbalances, which are defined as the difference of the prismatic powers in the corresponding visual points of the right and the left spectacle lenses, increase strongly with the increase of the viewing angle, because the prismatic secondary powers are a function in a first approximation of the viewing angle (distance c from the optical centerpoint) and the optical power D according to the so-called Prentice formula:ΔP=c*(DR−DL)  (1)
DR referring to the dioptric action of the right spectacle lens and DL referring to the dioptric action of the left spectacle lens.
For single-vision lenses, it is at least possible to set both the prismatic power and also the equivalent power and the astigmatism optimally in the main visual point. For progressive lenses (progressive power lenses), in contrast, this is no longer possible in the reference points (far and near reference points), because they lie outside the optical center.
To counteract this problem, progressive lenses are therefore often provided with a so-called slab-off grind. For this purpose, the surface is inclined in a spectacle lens along a horizontal line which typically runs through the optical centerpoint. The prismatic power in the upper half of the spectacle lens (i.e., above the horizontal line) may thus be set differently than in the lower half of the spectacle lens (i.e., below the horizontal line), so that an equalization of the prismatic power on the right and left may occur.
However, this method has the disadvantage that an image jump occurs along the horizontal line because of the discontinuity in the first derivative. A further disadvantage is that the partition line is visible and thus cosmetically unfavorable. In addition, only a correction of the vertical prismatic components in one point may be achieved using this method.
For the optimization of typical progressive lenses, it has been suggested that the binocular vision be improved by a minimization of the difference of the second-order imaging errors (i.e., the equivalent error and/or the refraction errors and the astigmatic errors) in the corresponding visual points of the right and the left spectacle lenses. Such a method is described, for example, in WO 01/46744.